1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed using a thin film transistor (hereinafter, referred to as TFT) and a manufacturing method thereof. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel, or an electronic device which has a light-emitting display device including an organic light-emitting element as a component.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics are a kind of compound semiconductor. The compound semiconductor is a semiconductor formed using two or more kinds of atoms bonded together. In general, metal oxides become insulators. However, it is known that metal oxides become semiconductors depending on the combination of elements included in the metal oxides.
For example, it is known that tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are metal oxides which have semiconductor characteristics. A thin film transistor in which a transparent semiconductor layer which is formed using such a metal oxide serves as a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (Patent Document 5 and Non-Patent Documents 5 and 6).
Further, attention has been drawn to a technique for manufacturing a thin film transistor using an oxide semiconductor, and applying the thin film transistor to an electronic device or an optical device. For example, Patent Document 6 and Patent Document 7 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a thin film transistor is used as a switching element or the like of an image display device.